1. Field of the Invention
The present invention relates to a semiconductor device, and particularly to a semiconductor device provided at its main surface with an alignment mark.
2. Description of the Background Art
A manufacturing process of a semiconductor device includes steps, in which positioning is performed by recognizing an alignment mark. For example, a process of manufacturing a semiconductor device with memory cells includes a step, in which a position of a fuse is determined for switching circuits. Many semiconductor devices with memory cells are provided in advance with spare cells because a failure may occur in some of the memory cells due to foreign material mixed thereinto during a manufacturing step. When the failure in cells is found, the circuit is switched to replace faulty memory cells with the spare cells. For this replacement, such a manner has been widely employed that a corresponding fuse formed in the circuit is blown to designate a specific cell address. A laser trimming manner, in which a laser beam is emitted for blowing, has been widely employed for blowing the fuse. For determining the position of the fuse, the semiconductor device is provided with alignment marks. By using the same laser as that used for the blowing, the alignment mark formed on the semiconductor device is scanned, and the alignment mark is recognized by sensing contrast between light reflected from a high reflectance portion and light reflected from a low reflectance portion around the high reflectance portion in the alignment mark.
Japanese Patent Laying-Open No. 2001-168194 has disclosed a semiconductor device with alignment marks, in which a low light reflectance region is formed on the same thin film as a fuse element. This semiconductor device is provided at the low light reflectance region with many dots each formed of a thin film of polycrystalline silicon, which is the same thin film as the fuse element. In another structure, a first insulating film formed of a silicon oxide film is formed on a silicon substrate, and a polycrystalline silicon thin film is partially formed in a dot-like form on the first insulating film. Unevenness is formed at a surface of an aluminum film, which is located above a region provided with the dot-like polycrystalline silicon film. This unevenness irregularly reflects light beams emitted thereto. This region serves as a low light reflectance region. Further, a portion of the aluminum film, which is located on a region not provided with the polycrystalline silicon thin film, has a flat surface, and can serve as a high light reflectance portion. As described above, the foregoing reference has disclosed an alignment pattern formed of the low and high light reflectance regions.
Japanese Patent Laying-Open No. 7-335721 has disclosed a semiconductor device having alignment marks, which provide a large difference in intensity of reflected light. The alignment mark has a flat portion reflecting laser beams vertically upward, and an uneven portion for irregularly reflecting the laser beams. To a vertically upper portion, the flat portion of the alignment mark can reflect the light of a high intensity, but the uneven portion can reflect only the light of a low intensity. This is because surfaces of the uneven portion irregularly reflect substantially all laser beams. In this manner, a large difference occurs in intensity of the reflected light so that the alignment mark can be recognized accurately.
For reduction in size and increase in operation speed of semiconductor devices in recent years, various new materials have been newly used for production of the semiconductor devices. For example, copper is used as an interconnection material for lowering a resistance value of interconnections and thereby increasing the operation speed, and a silicon nitride film or the like is formed on an upper surface of such an interconnection layer for the purpose of preventing diffusion of the copper from the interconnection layer. A material of a lower dielectric constant than a conventional interlayer material is being used as a material between the interconnection layers. These new materials have been newly used for achieving an intended performance of the semiconductor devices. Therefore, optical characteristics-such as reflection characteristics and refraction characteristics of these materials are not necessarily optimum for scanning of the alignment marks with laser beams.
This causes a problem when a predetermined alignment mark is to be recognized with a light source, e.g., of laser in a manufacturing process of a semiconductor device. Thus, such a problem occurs that the mark cannot be recognized without difficulty because large noises coming from a portion around the intended mark are added to the light beams reflected by the mark. The alignment mark is basically formed of high and low reflectance portions. By scanning the mark with laser beams, a reflection waveform is obtained. This reflection waveform is formed of a portion of light of a large intensity primarily reflected by the high reflectance portion and a portion of light of a low intensity primarily, which is reflected by the low reflectance portion and a layer boundary immediately under the low reflectance portion. If all insulating films layered within the semiconductor device are made of silicon oxide films, a difference in refractive index between the layered films is very small. Therefore, laser beams incident on the low reflectance portion passes through the respective insulating films made of the silicon oxide films. The laser beams reflected from the low reflectance portion have a low intensity, and can provide good reflection waveforms, which allow easy discrimination between portions of high and low light intensities. However, if insulating films such as silicon nitride films or silicon carbide films are added to the insulating films of silicon oxide films as already described, a difference occur in refractive index between the insulating films. For example, if the silicon oxide film and silicon nitride film are layered together, the neighboring films represent different refractive indexes, respectively. Thereby, a boundary surface between the silicon oxide film and silicon nitride film partially reflects the incoming laser beams so that the laser beams returning from the low reflectance portion have a relatively high intensity. In particular, if the semiconductor device is internally provided with many interconnection layers, laser beams are reflected by the surfaces of the interconnection layers to a large extent. This reduces a difference in intensity between the light reflected from the high reflectance portion and the light reflected from the low reflectance portion so that the position of the alignment mark cannot be accurately determined.
Meanwhile, semiconductor devices employing more layers have been developed and employed and, in the field of system LSIs (Large Scale Integrated Circuits), semiconductor devices internally provided with six or more interconnection layers have been increasing. With increase in number of the layers, variations in total thickness of the multi-layer structure may increase due to summing of variations in film thicknesses of the respective layers. Depending on these variations, variations occur in intensity of light reflected from the low reflectance portion, resulting in unstable recognition of the alignment mark. In particular, required accuracy of positioning in a fuse blowing step has been increasing in accordance with miniaturization of semiconductor devices, but difficulty in maintaining intended accuracy of recognizing the alignment mark has been increasing due to formation of the insulating films of various materials, and thus due to variations in component material,